A fuel cell, as presented in FIG. 1, includes an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs) oxygen 106 is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with fuel 108 producing water and also for example, carbon dioxide (CO2). Between anode 100 and cathode 102 is an external electric circuit 111 having a load 110 for the fuel cell.
FIG. 2 shows a SOFC device as an example of a high temperature fuel cell device. A SOFC device can utilize as fuel for example natural gas, bio gas, methanol or other compounds containing hydrocarbons or pure hydrogen. A SOFC device in FIG. 2 can include more than one, for example, plural fuel cells in stack formation 103 (SOFC stack). Each fuel cell includes an anode 100 and cathode 102 structure as presented in FIG. 1. Part of the used fuel can be recirculated in feedback arrangement 109 through each anode. A SOFC device in FIG. 2 also includes fuel heat exchanger 105 and reformer 107. Several heat exchangers can be used for controlling thermal conditions at different locations in a fuel cell process. Reformer 107 is a device that converts the fuel such as for example natural gas to a composition suitable for fuel cells, for example to a composition containing hydrogen and methane, carbon dioxide, carbon monoxide and inert gases. However, in each SOFC device it is not necessary to have a reformer.
By using measurement means 115 (such as fuel flow meter, current meter and/or temperature meter) desired and/or necessary measurements are carried out for the operation of the SOFC device. A fraction of the gas used at anodes 100 may be recirculated through anodes in feedback arrangement 109 and the other part of the gas is exhausted 114 from the anodes 100.
A solid oxide fuel cell (SOFC) device is an electrochemical conversion device that produces electricity directly by oxidizing fuel. A SOFC device can provide high efficiencies, long term stability, low emissions, and reduced cost. However, such devices can have high operating temperature which results in long start up and shutdown times and in both mechanical and chemical compatibility issues.
Natural gases such as methane and gases containing higher carbon compounds have been used as fuels in SOFCs, which gases, however, have to be pre-processed before feeding to the fuel cells to prevent coking (i.e., formation of harmful carbon compounds such as for example coke, fly dust, tar, carbonate and carbide compounds). These different forms of carbon can be in this context referenced to as generally being harmful carbon compounds. Hydrocarbons go through a thermal or catalytic decomposition in the formation of harmful carbon compounds. The produced compound can adhere to the surfaces of the fuel cell device and adsorbs on catalysts, such as nickel particles. The harmful carbon compound produced in the coking coats some of the active surface of the fuel cell device, thus significantly deteriorating the reactivity of the fuel cell process. The harmful carbon compounds may even completely block the fuel passage.
Preventing formation of harmful carbon compounds can, therefore, be important for ensuring a long service life for the fuel cells. The prevention of formation of harmful carbon compounds also saves catalysts that are the substances (nickel, platinum, etc.) used in fuel cells for accelerating chemical reactions. Gas pre-processing requires water, which is supplied to the fuel cell device. Water produced by combining oxygen ions and fuel (i.e., the gas on the anode 100 side), can also be used in the pre-processing of the gas.
The anode electrode of solid oxide fuel cell (SOFC) can contain significant amounts of nickel that is vulnerable to form nickel oxide if the atmosphere is not reducing. If nickel oxide formation is severe, the morphology of electrode material may change irreversibly causing significant loss of electrochemical activity or even break-down of cells. Hence, SOFC systems require protective atmosphere, such as gas containing reductive agents during the start-up and shut-down in order to prevent the fuel cell's anode electrodes from oxidizing. In practical systems the amount of purge gas has to be minimized due to cost and storage space reasons. Purge gases are not necessarily elemental and they can be also compound gases.
Known processing of SR (Steam reforming) in fuel cell systems produces carbon dioxide CO2 and hydrogen H2 and excess steam. Requirements for fuel cell system start-up or shutdown operation includes sufficient steam and hydrogen production, and a SR process can be used when an external water supply or water container is available, as well as a related purification system, evaporator, water supply equipment and other equipment required to generate the start-up steam. Required peripherals used for start-up steam generation increase the system cost and decrease reliability due to increased complexity. In larger systems the methodology can include recirculation of part of the anode exhaust back to the reformer inlet to recover steam generated in the fuel cell reactions to be used in the steam reforming process, thus reducing or eliminating the need for continuous external water feed. However, when load is not applied to the fuel cells, such as during start-up, shutdown or idling, steam formation at the fuel cells does not occur.
Processing of CPOx (Catalytic Partial Oxidation) in fuel cell systems traditionally produces carbonmono-oxide CO and hydrogen H2. Requirements for fuel cell system start-up or shutdown operation includes sufficient steam and hydrogen production, where CO production in a larger amount is harmful. Using higher air (i.e., oxygen) amounts for more complete oxidation produces too much heat making temperature rise excessive in the start-up situation or cooling process too slow in the shutdown situation.
Processing of OSR (Oxygen-Steam reforming) in fuel cell systems is a combination of both CPOx and SR (Steam Reforming), where both air and steam are supplied to the reformer and are known to produce carbon dioxide CO2 and hydrogen H2 and excess steam. Requirements for fuel cell system start-up or shutdown operation includes sufficient steam and hydrogen production, and an OSR process can be used when an external water supply or water container is available for SR, as well as a related purification system, evaporator, water supply equipment and other equipment required to generate the start-up steam, and a supply of air or other source of free oxygen is available for CPOx. Required peripherals used for start-up steam generation increase the system cost and decrease reliability due to increased complexity.
CPOx (Catalytic Partial Oxidation) can produce carbon monoxide CO and hydrogen H2. This gas mixture is used for various chemical industry purposes, and the operating temperature of CPOx is above 700° C. The known product gas is unsuitable for fuel cell due to coke formation in the system heating/operating temperatures. Requirements for start-up or shutdown gas include sufficient steam and hydrogen production, whereas CO production in larger amount is harmful. Using higher air (i.e., oxygen) amounts for more complete oxidation, produces too much heat making the temperature rise excessive in regards to normal SOFC operating conditions, thermal management, thermal stresses and material selection. A method to increase the amount of oxidation without excessive temperature rise is to perform partial oxidation in multiple stages with intermediate cooling prior to feed of additional oxygen. Such arrangements increase the cost and complexity of systems.
Known documents relating to this technical field include patent application document US 2011/159386 A1 which discloses a process for starting up a fuel cell system, which has a fuel cell with a cathode side and an anode side, a reformer and an auxiliary burner. Fuel cell air is preheated with the auxiliary burner and fed to the cathode side of the fuel cell. Residual gas is circulated from the anode side of the fuel cell to the reformer and from the reformer to the anode side. In this publication, the reformer is heated by overstoichiometric combustion of fuel (burner operating phase) whereby the reformer outlet gas is bypassed from the anodes. This includes costly flow diverting under high (˜900° C.) temperature. During the burner operating phase anode circulation is not active. Furthermore, the document discloses that reformer operation is temporarily started when anodes are below 250° C., which poses a risk of forming hazardous nickel carbonlyl compounds. US2011159386 does not present an operating mode in which air to fuel ratio is over 0.55 and temperature management of the reforming reaction is handled by recirculation. Hence, the embodiments of US2011159386 do not present a solution to provide safe operating conditions to fuel cells in all conditions while limiting the temperature rise in the reformer with minimum amount of system complexity.
Patent application document US2006093879 A1 discloses a procedure for starting up a fuel cell system having an anode exhaust recycle loop. The fuel cell system is disconnected from its primary load and has air in both its cathode side and anode side. A major part of gas from recirculation of the anode side flow is exhausted and only a small limited flow of hydrogen is provided into the anode side recirculation. Hydrogen and oxygen in the fuel and air mixture are catalytically reacted as they recirculate in the anode side until substantially no oxygen remains in the recycle loop; and then the fuel flow rate into the anode side flow is increased to normal operating levels and thereafter connecting the primary load across the cell. Embodiments presented in US 2006/093879 A1 are for a reforming stage of the fuel cell system, and the in this document discloses a system intended to remove oxygen from the anode side. Hydrogen and water steam have to be fed to the anode side, instead of production of them.
Patent application US 2002/102443 A1 discloses a procedure for shutting down a fuel cell system having an anode exhaust recycle loop. In the embodiments of US2002102443 a resistive load is connected in parallel with the fuel cell to limit the voltage and to react residual hydrogen. This approach in a SOFC would cause irreversible anode oxidation due to fuel shortage. A portion of the anode side flow exhaust is recirculated through the anode side in a recycle loop during operation. The fuel cell system is shut down by disconnecting the primary load from the external circuit and thereafter stopping the flow of fresh hydrogen containing fuel into the anode side flow and catalytically reacting hydrogen in the anode side recirculation by recirculating such gases within the anode recycle loop into contact with a catalyst until substantially all the hydrogen is removed. Thus US 2002/102443 A1 presents a similar but reversed method to that presented in the document US 2006/093879 A1.
Patent application document WO 2013/117810 discloses an arrangement utilizing recirculation for high temperature fuel cell system. In the embodiments of WO2013/117810 catalytic partial oxidation in the recirculation flow disclosed to produce a partially oxidized start-up gas with a substantially low amount of carbon monoxide for the recirculation flow in fuel cell system start-up or shutdown situations by exhausting 30% or less of the entire flow from the anode outlet flow. The embodiments of WO2013/117810 are restricted to catalytic partial oxidation operation, and suffer from rather strict requirements on operation conditions. The generated inert gas contains a potentially insufficient amount of hydrogen due to high lambda values. Also, due to high lambda values, heat control of the reformer requires active cooling and/or very high recirculation rates. WO2013/117810 does not present a procedure according to which safe operation with respect to carbonyl formation in start-up and shutdown can be achieved without risk of anode oxidation.